This invention relates generally to improvements in ceramic filter capacitors of the type used for decoupling undesired interference signals from implantable medical devices such as defibrillators and pacemakers. More particularly, the invention relates to an improved feedthrough ceramic filter capacitor.
As typically constructed, a feedthrough filter capacitor used to suppress undesired electromagnetic interference (xe2x80x9cEMIxe2x80x9d) signals along a pacemaker or defibrillator electrical lead has two or more sets of electrode plates embedded in a stacked spaced relationship within an insulative substrate. The stacked plates alternate between electrically positive plates and electrically negative or ground plates. Generally, the positive electrode plates are connected in parallel to the medical device electrical lead utilized to pass the desired electrical signals. In operation, the filter capacitor permits the passage of the relatively low frequency desired electrical signals, while shunting the relatively high frequency undesired electrical signals to a ground which is typically the conductive housing of the implanted medial device. Such devices typically have conductive housings formed from a biocompatible conductive alloy such as titanium. Preferably, the ground plates of the filter capacitor are directly coupled to the medical device housing to ensure against the entry of EMI signals to the interior of the device housing. The entry of any undesired EMI signals to the interior of the pacemaker, defibrillator, or other medical device may adversely effect the functioning of the device with significant adverse consequences to the health of the patient.
Multilayer ceramic capacitors of the type described above consist of a plurality of interleaved layers of conductive film and dielectric material. The conductive layers are formed by the deposition of a thick film metal paste or ink, commonly referred to as the electrode composition, over previously formed dielectric layers of ceramic oxide material.
The electrode composition is usually a dispersion of finely divided precious metal powders such as palladium, silver, gold, or platinum and mixtures thereof, in a binder, which is usually solely organic in nature. Dispersions of non-precious metals such as copper and nickel have also been found to have utility in electrode compositions. The binder is usually composed of a mixture of a polymeric resin which imparts viscosity to the composition and appropriate solvents for processing compatibility, particularly with respect to drying. Typical electrode composition metal concentrations range from about 40% to about 70% by weight, with the remainder being binder.
The dielectric layer is usually composed of finely divided oxide powders dispersed in a resin. Barium titanate and other oxides such as neodymium titanate and magnesium titanate are used. Additions are usually made to these oxides to control various electrical characteristics and particularly to maximize the dielectric constant. The resin is also present in the dielectric layer to facilitate the handling and deposition of electrodes on the layers.
Multilayer ceramic capacitors are manufactured by building up an interleaved configuration of electrode and dielectric layers, dicing individual parts out of the buildup and then subjecting the parts to a slow burnout or curing, then high temperature sintering. Burnout is done to remove the organic resin in the electrode and dielectric layers to avoid rapid outgassing and rupture of the layers. Sintering is done to a peak temperature to both densify the dielectric material to achieve physical strength, and to react the chemical constituents of the dielectric such that the desired electrical characteristics are achieved. During the sintering step, the powder grains in the electrode layers also densify so as to produce a continuous, electrically conductive metal film.
One common process for forming the interleaved structure of metal film and dielectric layers is referred to as the xe2x80x9cdry stack process.xe2x80x9d In the dry stack process, the dielectric is cast into a tape from a slurry of dielectric powder, solvents, and pliable resins. An electrode layer is then deposited on the tape, typically using a screen printing process. Another layer of tape is placed over the dried electrode print, a subsequent electrode is printed on the tape, dried, and another tape is placed. This process is repeated until the number of layers desired is achieved. Once completed, metal ink/dielectric layers are cured and sintered as described above.
Although, ceramic capacitors of the type described above are in widespread use as filter capacitors for implantable medical devices, such capacitors have limited current carrying capability are therefore limited to parallel connections with the leads from the implanted medical device. However, recent advances in pacemaker and defibrillator technology have created a need for capacitors with enhanced EMI filtering capability. To achieve such capability it is necessary to connect the filter capacitor in series with the leads from the implanted medical devices. However, series connection creates an additional problem in that the series connected capacitor must be capable of carrying the full current load of the lead to which it is connected. Capacitors of prior art design are not able to carry the necessary currents in a package size small enough to be used with implantable pacemakers and defibrillators. Thus, there exists a need for a high current feedthrough capacitor that may be connected in series with the leads of implantable medical devices.
The present invention is an improved monolithic multilayer ceramic filter capacitor for use with implanted medical devices such as pacemakers and defibrillators. The new capacitor includes a parallel stack of positive electrode plates that, when combined, have sufficient current carrying capability such that they may be directly connected in series with a lead wire from an implanted medical device. Thus, the present invention capacitor may be used for both filtering and pulsed energy transmission from a device such as a defibrillator. This is in marked contrast with prior art capacitors which may be used for signal filtering only.
The present invention capacitor is capable of carrying high current through the use of interleaved electrode plates where the positive electrodes are connected in parallel, and where at least one of the positive electrodes is of sufficiently high conductor density to carry a majority of the current transmitted by the implantable medical device lead. Preferably, the high conductor density electrode is an external or surface electrode. With this design, the present invention capacitor may be produced in a smaller package than a conventional ceramic capacitor of equivalent current rating. Through the use of a high density surface electrode, the present invention capacitor allows for an increase in the number of internal electrode plates, thereby providing for increased capacitance and greater signal filtering capability, than would be achievable with a conventional ceramic capacitor of the same physical size and similar current carrying capacity. Other features and advantages of the invention will become more apparent from the following detailed description of the invention, when taken in conjunction with the accompanying exemplary drawings.